Voltage-Source Converters (VSC) for High-Voltage Direct-Current (HVDC) systems offer a cost-effective solution for interconnections in or between power networks, allowing for active and reactive power control in both directions, while preventing fault propagation and generally increasing low-frequency and voltage stability. The main control challenge for the operation of VSC-HVDC systems is achieving a stable and reliable operation over a wide range of operating conditions and system parameters. Moreover, suppression of transient over-voltages and over-currents is crucial for the operation of the system. Known control solutions for VSC in HVDC applications are based on the design of controllers using locally linearized and/or averaged system models which are barely capable of predicting or analyzing the emergence of local power system instabilities, since the dynamics which cause the latter are not properly accounted for in the controller design.
The paper “Dynamic Model and Predictive Current Control of Voltage Source Converter Based HVDC” by Xiaoyan Wen et al., IEEE International Conference on Power System Technology 2006 (XP031053573), proposes a control method for a VSC-HVDC with a slow “external” and a fast “internal” control loop. The external or outer control loop controls the DC link voltage and/or the active and reactive power flows by determining the two converter current component references (iα, iβ) in a rotating reference frame as input to the internal or inner control loop. The latter calculates the voltages used to track the requested currents, for example, modulation voltages uα, uβ are determined and ultimately fed into a Pulse Width Modulator (PWM) unit. The internal loop is designated as a “predictive current control” and assumes that the desired current reference will be attained with a simple one-step-ahead prediction. The external loop features a phase-locked loop (PLL) to transform (into the rotating reference frame) and decouple the state variables so that the adopted PI (Proportional-Integral) control scheme can operate correctly.